Breath of the Sun
by tuntuntunaa
Summary: In which Kamado Tanjirou was summoned as Saber in the Fifth Holy Grail War, and single-handedly ended the War by being a nice person and a really good big brother.


_**In which Kamado Tanjirou was summoned as Saber in the Fifth Holy Grail War and single-handedly ended the War just by being a nice person and a really good big brother. **_

**This story follows F/SN:UBW timeline, though it will deviate from canon.**

**Warning: One or two spoilers from KnY manga, though I made up the ending. KnY manga was yet to end when this was written (29/02/2020). Honestly, you could still read this without having read the manga beforehand. **

**Also, at the start of UBW, Shirou is still somewhat naive.**

**There will be a lot of timeskips, because this is supposed to be a two/three-shots. Anything not important from the anime will be skipped.**

**See the Author Notes at the bottom of the page for some explanations.**

**Well, then. Enjoy!**

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**Disclaimer: I do not own FSN nor KnY.**

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**Breath of the Sun**

Fate/Stay Night: Unlimited Blade Works x Demon Slayer: Kimetsu no Yaiba crossover

Un-betaed

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**Part I - Prologue**

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Emiya Shirou couldn't remember his life before the fire.

His body, however, _remembered._

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"...What are you doing, son?"

Shirou paused, the karage millimeters away from his agape mouth.

"...I'm eating?" He said uncertainly to his foster father. "What's wrong?"

Kiritsugu stared at him in silence. Shirou put down his chopsticks and shifted nervously as the seconds ticked by. He checked himself over to find nothing wrong, before thinking hard whether he'd done something he wasn't supposed to do in the past few days.

"Uh, I know I shouldn't practice magecraft-"

"You're breathing strangely, Shirou-"

Both father and son spoke at the same time. There was an awkward pause, before Kiritsugu cleared his throat.

"Your breathing is irregular, yet strangely rhythmic," he said. "Sometimes you breathed quickly as if you were purposely trying to hyperventilate. Other times, you didn't take a breath for longer than a minute. I can understand if you're doing that when you're meditating, but not really when you're hyperventilating yourself during every meal."

Shirou blinked, taken aback. "You mean, you don't?"

Kiritsugu's eyebrows rose. "No, I don't. Pretty sure most people don't."

"Huh," Shirou picked up his chopsticks again and started eating. He barely realised his breathing quickening automatically. "Well. Nothing has gone wrong so far, Dad, so I guess I'm fine?"

He could feel Kiritsugu's gaze on him for another minute before his father sighed. "You're right. Wouldn't hurt to learn breathing techniques if you know what you're doing."

Shirou beamed at him before focusing back on his meal.

"You should do it too, Dad," the young boy said idly as he scooped more rice from the rice cooker to his own bowl. "You haven't been feeling well lately. I could teach you some to help you calm your body."

Kiritsugu blinked. "I'm surprised you could remember that. I thought you don't remember anything from before the fire."

"I don't," Shirou shrugged. "But I've been doing this for as long as I could remember. It's like my body does it automatically. I think I even do it in my sleep."

"No wonder you're such a calm kid…," Kiritsugu mumbled to himself. He gave him a tiny smile then, "Alright, teach me how, son."

Shirou nodded excitedly. "Okay!"

"After telling me _what_ exactly have you been practicing _without supervision_."

He gulped.

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* * *

Shirou was in his first year of middle school when he realised that his body was stronger and faster than his peers'.

"What the - your head's fucking hard!" The bully cursed loudly, cradling his hands. "Shit! I think you broke my bones!"

"...Uh, sorry?" Shirou offered awkwardly. The bully had punched his face so technically he should be the one apologising, but Shirou didn't expect said punch to break the bully's hand instead. "You should go to the hospital."

The bully's friends had other priorities, apparently. They charged at the lone redhead, intent to beat him to a pulp.

Shirou dodged a kick (because he didn't want to accidentally break more bones) and lightly slapped someone's wrist to redirect the punch. Unbeknown to him, his body automatically performed an appropriate breathing technique to strengthen his body during combat, enhancing his base strength, speed and reflex.

Thus, instead of redirecting the punch, Shirou accidentally dislocated the bully's shoulder.

(Since then, all bullies ran away whenever they saw him.)

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* * *

"Emiya-kun, have you done archery before?"

Shirou blinked, before relaxing his stance, bow in his side. He turned to the archery captain and said, "No. Why?"

The captain frowned at him, before shrugging. "Your breathing is good, Emiya-kun. Really good. Go on, shoot."

His first try, _ever_, got him a bullseye.

"...Must be a fluke," someone in the crowd muttered.

Shirou's subsequent attempts earned him more bullseyes.

"What the - that's it. You're in!"

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* * *

Sakura peered out of the window, silently watching the boy practicing high jump in the field.

She didn't know who he was, but she had seen him around the school before. The redhead was her brother's close friend, thus someone who she should really avoid.

Except, he was different. Her brother wouldn't bother to practice such a mundane sport like this. But the boy was persistent, persevering despite his constant failure, and Sakura admired him for that.

It was twenty minutes of futile practice when the boy finally stopped. Sakura was about to leave the premise when she realised that he was not done yet.

He began his running start again, but this time Sakura noticed...mist? Coming out of his mouth as he exhaled. She leaned forward in anticipation when the boy jumped.

He jumped and managed to successfully land on the other side of the pole.

Problem was, he jumped _three_ meters above the ground.

Sakura gaped at him as he quickly stood, dusted himself off before scanning his surroundings. His eyes met her and he froze. Blushing, he scurried away in what she thought would be an embarrassment.

"...Magecraft?"

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* * *

**Part II - Day 1**

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"I have come to answer your call," a deep voice resounded in the small warehouse, breath whistling as the man exhaled and turned around. "I ask you; are you my Master?"

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* * *

The fire was so unexpected it burned his hands slightly.

"Archer!" Rin screamed in alarm as an unknown swordsman appeared in front of them both.

It was then Archer realised it was not _her_. Instead of Arturia Pendragon, there stood a tall Japanese man with long burgundy hair tied back in a ponytail and eyes of the same shade. A dark, distinctive scar was prominent on the left side of his forehead, and a pair of hanafuda earrings dangled in his ears. He wore a black gakuran set beneath a green, checkered haori. And in his hands, a _sword_ -

Archer took a sharp intake of breath.

"...What a magnificent blade you have there, Kamado Tanjirou," he finally said, nodding at the crimson Nichirin Blade.

"What?" Rin turned to him in surprise. "How did you know his name?"

The swordsman, Tanjirou, blinked at him in surprise too. His nose twitched and his eyes widened slightly, before relaxing his stance.

Rin was baffled, especially when her own Servant did the same.

"...I didn't expect to see my descendants when I was summoned here," Tanjirou smiled warmly. It was such a great contrast to Lancer's bloodlusted grin in their short fight moments before, that Archer and Rin were stunned to silence. Or maybe the 'descendants' part was so shocking that they were left speechless. "Two of them, in fact. As expected of the Holy Grail."

"S-Saber!"

Then, Emiya Shirou emerged from the gate of his house.

A lot of things happened at the same time, then.

Shirou skidded into a halt. "Huh? Tohsaka?"

Rin reeled back at the redhead's appearance. "Emiya-kun?! You - what - how come you're here?! Wait, you're a Master?!"

Archer was torn between glaring at his past-self and gaping at the Servant Saber in front of him. The Nichirin sword did provide him the history of the sword, but it did not tell him anything else beyond the death of the swordsman himself. And it was Kamado Tanjirou his past-self had summoned, the famed Fire Pillar who'd ended the demonic reign of Kibutsuji Muzan in the Taishou era.

The famous Demon Slayer hero, who was apparently _his_ ancestor. In Archer's whole life, how come he never knew about this?!

Tanjirou looked back and forth, between his Master and presumably the Servant Archer, brows furrowing in confusion.

"Huh, now that you are here...how come you two have the same scent?"

(Then and there, Archer's true identity was revealed. Apparently, Tanjirou's sense of smell was much stronger and more versatile than Emiya Shirou's.)

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* * *

"Oh my god."

Pink eyes blinked back at her.

"S-s-s-," Rin stuttered as she trembled violently, rooted to the spot. Finally, she gasped, "_So cute!_"

The usually impeccable girl dove to hug the tiny cute thing and rubbed her cheek on the thing's head.

"Wha- who is this?" Shirou crouched down to stare at the little girl who appeared out of nowhere. She was probably around five years old, with pink eyes and long dark hair with vermilion tips. She wore a light pink kimono underneath a long dark brown haori. Strangely, there was a bamboo gag on her mouth.

"Ah, that would be Nezuko, my little sister," Saber beamed, love and pride shining in his eyes.

Rin blinked, before pulling back to frown at the Servant. "...Wait, you're summoned together? Is she also a Servant?"

Saber shrugged. "Some may call her one of my Noble Phantasms, but I wouldn't call her that. Nezuko is Nezuko, my younger sister. She'd been an integral part of my life, so we're practically inseparable."

It was then Archer materialised. "...She's a demon."

The two Masters jerked up in shock, whirling to look more closely at the tiny girl.

"That's right," Saber smiled sadly. He patted Nezuko's head as the girl moved to sit on his lap. "Nezuko was turned into a demon by Kibutsuji Muzan. But she's the only demon who has never eaten a single human before, and the only one who managed to turn back to being human."

Shirou peered at the demon child. "Is she a human now?"

Saber shook his head. "I am summoned here as a version of myself at my peak. Nezuko's peak was when she was still a demon."

There was a comfortable silence as they enjoyed the snacks and tea Shirou had provided. After learning Archer's true identity, Rin decided to form a truce with the Master of Saber. All of them were too shocked to do anything else much, besides taking the wait and see approach. Plus, Rin knew that Archer was very curious about his ancestor despite him denying it. Nezuko was here too, so a truce was definitely beneficial.

"Ah, she's sleeping," Rin cooed at her. "Goodness, a demon shouldn't be this adorable…"

"I know right?" Saber laughed as he continued patting his sister's head softly. "Nezuko was known as a great beauty, both in my hometown and amongst demon slayers."

Suddenly, Shirou came to a realisation. "Could she be my great-grandmother?"

Saber stared at him for a minute, before deadpanning, "Nah. You look too blockheaded to be Zenitsu's great-grandchild. Must be mine."

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* * *

Tanjirou stood silently as Shirou and Rin talked to the priest, eyes scanning his surroundings.

'The smell here...It's bad,' his nose twitched in disgust. He zeroed in on the door leading to the church's basement, where the scent of pain, fear and hopelessness originated from. Tanjirou gritted his teeth - there was someone, no, _people _down there, suffering for some reason.

And the worst thing was, he _can't _do anything about it. Not yet.

There was a strong scent of a powerful being emerging from the same door. It saturated the entire church compound, even more so than the disgusting smell coming from the fake priest (how could a human have a scent like this? How could a _degenerate _live amongst civilians like this?). This tainted scent of a Servant prevented Tanjirou from doing anything, lest he risked the lives of the two Masters.

No, he had to wait. Gritting his teeth, he forced himself to stay still.

'I will save you,' Tanjirou vowed to himself. 'Soon.'

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* * *

'That man...He's a lot worse than me,' Archer thought, irritated. "What did you just say?"

"I said, let me talk to her first," Saber repeated firmly. "Her scent...She's angry, but she's also lonely and in pain."

Rin threw her hands up in frustration. "That girl is an Einzbern! And that Servant is a Berseker, with stats much higher than yours! We can't afford to have a chat with her!"

"Still, let me talk to her," Saber said resolutely, before turning to face Emiya Shirou. "Master."

The brat, being another bleeding heart, nodded at him. "If you say so, Saber. Besides, it's best to avoid bloodbath as much as we could."

God, this was the worst pair of Master-Servant Archer had ever seen. Both were absolutely incorrigible, one more so than the other. Archer never thought he'd see someone much more fucked up than his past-self.

(And the worst part was, it _worked.)_

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**Part III - Day 2**

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"_My job is to slay demons. Needless to say, I'm going to decapitate your sister."_

"_Hold on! My sister hasn't killed anyone!"_

"_She can't return back to normal! Once you become a demon, you can never go back to being human."_

"_I'll find a way! So please...Please don't kill her! I'll make everything right!"_

That night, Shirou dreamed of a boy, weak and hopeless, begging for the Demon Slayer to let his sister go. It was the worst day of his life; his family slaughtered and his little sister turned into a monster. The Demon Slayer raised his katana to stab his sister's shoulder, and the boy screamed.

"_I'm begging you, please...She's the only one I have left."_

The Demon Slayer called him pathetic, groveling in the snow and willingly baring his life to death, too weak to see the reality.

_Guilt. Guilt. Guilt._

'Ah…,' Shirou thought distantly as he watched the boy desperately fight someone much stronger than him for the sake of others. 'He's just like me.'

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* * *

It quickly became apparent that Saber was just too nice of a person.

"Teach me your Sun Breathing Style," Archer said out of the blue.

"Sure," Saber beamed, uncaring about the fact that he shouldn't teach his enemy _his prized technique_.

(Archer felt a tiny bit guilty for exploiting the Demon Slayer's kindness like this.)

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* * *

Shirou found Saber in the doujou, punching Archer's solar plexus so hard the red Servant doubled over in pain. He swore he could hear the supersonic 'boom' from the impact.

"You're doing it wrong," Saber said kindly as if he didn't just attempt to kill Archer. "Again."

Surprisingly, Archer obeyed and went back to his stance.

"What are they doing?" Shirou jolted up when Rin suddenly appeared behind him. "Beating each other up?"

"No...More like Saber beating Archer up," Shirou said, before turning to the girl nervously. "I swear I didn't order Saber to do this."

"Oh, you're here, Shirou-kun," Saber waved at him. "I noticed you're doing breathing techniques all the time since we met. That's good. You should join us. I can teach you some stuff."

Shirou eyed the twitching mess on the floor and gulped.

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* * *

Sakura didn't expect to see an entourage when Fujimura-sensei told them Senpai was coming to deliver her lunch. Let alone people with such a high density of magic in them.

"Sorry for intruding," a tall man with long burgundy hair bowed slightly.

"Ooh, so this is the archery doujou," the white-haired girl Sakura swore must be the Einzbern Master, peered at the archery members in wonder. "Interesting! Hey onii-chan, can I try?"

"When you're older and a bit taller, Ilya," Senpai said, before turning to Sakura. "Is Fuji-nee here?"

"...Huh?" Sakura blinked at him for a moment, too caught up staring at another tiny girl with long dark hair exploring the entrance of the doujou. "Oh! Right, I'll get her."

When she came back with Fujimura-sensei, Mitsuzuri-senpai was chatting with Senpai and the tall man while she hugged the little dark-haired girl close.

Ignoring her blushing teacher at the sight of the tall man, Sakura looked around to find one missing person.

'Where is the Einzbern?'

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* * *

"Don't run off by yourself like that again, Ilya," Shirou sighed in exasperation. "You're lucky Kuzuki-sensei found you and called Fuji-nee."

"Okay!" Ilya hummed, looking totally unrepentant. "By the way, you and the people around you will die if you don't do anything, mister."

Shirou blinked. "What do you mean?"

"This school? There's a Bounded Field, meaning something big will happen here sooner or later. Your Kuzuki-sensei? I can sense magic in him, though not much. But I can tell you that he's a Master," Ilya suddenly revealed. "Your Sakura? Now _that _is a magus. As expected of the Matous. I'll be surprised if she's not a Master already."

Shirou was stunned speechless.

"I've noticed as well," Saber said, before smiling at him. "I can't do anything about the Bounded Field since I have no proper knowledge of magecraft. But don't worry about Sakura-chan. I can smell no hostility from her. She likes you too much to put you in harm."

"Huh?" Shirou took a moment to realise the implication. He blushed. "Um, right. Good to know."

"Though, her scent…," Saber muttered, frowning. "She's constantly in pain."

"That might be because of the Matou's magic," Ilya hummed in thought. "Matou Zouken's signature magic lingers in her body."

"Not only that," Saber clenched his fist in anger.

Shirou watched their reactions, before saying, "Alright, I'll be careful. As of Sakura, I'll talk to her brother tomorrow. Shinji might know something."

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* * *

"You're a strong girl, Sakura-chan."

Sakura paused mid-step. She was about to follow Senpai to the kitchen when he'd said that. "Tanjirou-san?"

Of course, Sakura knew this man was a Servant. She could sense the controlled power and magic in him. Whether Tanjirou was his real name or not did not concern her. Her Senpai's safety was the only one that mattered, so she was glad to know that this person was strong.

But she wasn't expecting any further interaction than exchanging polite bows.

"I know not of the exact suffering you have to go through in your own home, Sakura-chan," Tanjirou said softly. Sakura tensed at that. "But despite all the pain that I could sense from you, you persevere and remain kind. I admire that."

He stood up and carried the tray of tea to the porch. "Come, sit here."

Sakura glanced back at her Senpai, who was busy washing the dishes from their dinner. She hesitantly followed the Servant.

When she sat on the cushion, Tanjirou unceremoniously dumped his little sister Nezuko on her lap.

"Here, you can have Nezuko while I talk. She's very huggable," Tanjirou smiled at her.

Sakura did just that, unsure as to what the man wanted from her. Indeed, Nezuko was very huggable, Sakura thought. Her warmth grounded her.

"I know that you're a Master, Matou Sakura," Tanjirou revealed suddenly.

Sakura tensed, hands clenching onto Nezuko tightly. Nezuko looked up at her and cooed soothingly.

But the Servant didn't do anything else other than sipping his tea.

"...You're a Servant," Sakura finally whispered. "You're wrong. I am not a Master."

"True," Tanjirou hummed in agreement. "But I could sense the residual scent of the Command Spells. It was as if they were transferred somewhere else instead of being completely used up. Though that's not what I want to talk about."

He was silent for a few minutes, enjoying his tea as he relaxed on the porch. Sakura peered at him curiously; the man had scars covering every inch of his skin. He must've fought a lot in the past, winning and losing constantly as he clashed with his enemies.

But she'd never seen a hardened man like him being so relaxed like this. Her grandfather, a veteran magus, was never satisfied with anything, constantly striving to prolong his life. Kuzuki-sensei was calm and collected, but Sakura had never seen him relaxing like Fujimura-sensei. Even Shirou, her kindhearted senpai, had never laughed as freely as this man, his life fully dedicated to living and achieving his goal to be a Hero of Justice.

But Kamado Tanjirou was strangely content. It was as if his wish had been fulfilled a long time ago, and he was now at peace.

"What is your goal, Tanjirou-san?" Sakura finally asked him. "The Grail must've summoned you for a reason."

"I wish to protect my family," he smiled and patted her head. "That includes anyone close to us. Shirou-kun may have been orphaned twice, but he has people around him that he could trust. You're one of them, thus you're family."

"You see, Shirou-kun is of my blood, which is why our souls could resonate a lot more than any other Master-Servant pair. I could understand his mindset now. It is similar to mine, a survivor thinking that his life is worth less than others. But I have Nezuko and my friends to rely on," Tanjirou sighed, and Sakura could hear sadness from his voice. "Unfortunately, Shirou-kun hasn't had a solid one ever since the fire."

"But I trust you, Sakura-chan," Tanjirou smiled warmly at her. He tapped his nose and continued, "My sense of smell is quite special. I could sense the emotion and nature of a person from their scent. Despite your pain and your occasional dark thoughts, I could sense your kindness and love, especially towards my kin. For that, I am grateful."

"...But I have done nothing worthy to be his pillar," Sakura looked down, eyes unseeing as she rubbed Nezuko's head softly. "I...I am unclean. Tainted. How can I be someone else's pillar while I myself am struggling to stay sane?"

"Then, it's a good thing that you could rely on him too," Tanjirou patted her shoulder as he made eye contact with her. "Be each other's pillars. Trust in him and ask for his help. Ground yourself through him. Don't ever let him go."

(_Don't let him go, and he'll be there when you need him the most._)

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* * *

Ilya observed Saber's interaction with his Noble Phantasm closely.

"She's my little sister, Nezuko," Saber had said to her last night, when Ilya had decided to stay in the Emiya residence for the night. "Hope you two could get along."

Ilya had frowned at him then, unsure by the way he treated her. "She's, what, five? I'm not a little girl."

Suddenly, the Noble Phantasm girl grew up, matching Ilya's appearance-age. Nezuko's eyes crinkled in a smile, and she patted Ilya's head. Saber laughed and did the same to both of their heads.

Their hands were warm.

That night, Ilya slept soundly for the first time since the day her parents left her behind, safely cocooned in the warm embrace of her new friend Nezuko.

(Is this how having a sibling feels like? Maybe I should tell him tomorrow.)

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* * *

The next night, Ilya did exactly that.

Shirou was silent for a few moments. Ilya waited patiently, crushing the hope which had blossomed in her heart since last night so that the inevitable rejection would not hurt as much.

But it never came.

Shirou kneeled in front of her. His hands rose to embrace her close.

_He's warm_.

"I'm sorry I didn't know sooner, Ilya," Shirou mumbled softly.

"It's okay," Ilya clung on him more tightly. "It's my fault I didn't tell you sooner."

_My hatred had almost blinded me. Almost robbed me of the only family I have left._

"Tell me more about Daddy later?"

"Of course."

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* * *

**Part IV - Day 3**

* * *

_No wasted breath, no unnecessary movement. Every step was meaningful._

_It was truly beautiful._

_Makoto and Sabito taught him how to wield his blade, to execute the Breath of Water perfectly._

_But they were already dead. The demon had claimed he'd eaten them. Master Urokodaki confirmed that._

_He wished he could do something more, but apparently, Master Urokodaki said it was enough that he was alive._

"_You must come back, Tanjirou," he'd said. "Don't leave your sister alone. Come back alive."_

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* * *

Shinji was absent today.

Shirou couldn't ask him about Sakura because of it. Though, when he bumped into him on his way to the school yesterday, Shinji was acting strangely. The blue-haired boy had sneered, hatred clear in his eyes, stunning Shirou to a halt.

Sure, he was nasty sometimes, but Shinji had never hated him. Shirou was the only one willing to be his friend (somewhat) because he knew that the boy was lonely. Besides, Sakura was his friend too and it would be best for Shirou to befriend her brother as well.

However, Saber's comment on Sakura's pain, as well as his Servant's interaction with his own little sister, made him realise that something was not quite right between the Matou siblings.

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* * *

Investigating Mitsuzuri's strange absence had turned into a game of cat and mouse between him and Tohsaka instead.

He didn't have his Servant with him because Saber could not dematerialise. It was late, the school was empty, and his temporary truce with Tohsaka has ended. It was a perfect opportunity for the Master of Archer to strike.

Shirou regulated his breathing and dodged, the curse whistling past him by a hair's breadth. The Gandr hit the wall and completely obliterated it.

"Stay put, you coward!" Rin hollered from afar.

"No way am I going to stay put and let myself be hit by _that_," Shirou grumbled to himself. Fortunately, he had his breathing techniques to rely on.

Now, to find a 'sword'...

A few moments of being trapped in her Bounded Field, bombarded by mini-Gandrs and bombed by a grenade-gem later, Shirou was prepared to fight Tohsaka with his broken pipe head on when he heard a scream.

"Wait, Emiya-kun!" Rin yelled at him when he immediately dashed towards the source of the scream.

Shirou found her between the main building and the club building. It was a student he didn't know, and she laid motionless on the floor, her skin pale and cold. Tohsaka shoved him aside, stating that her blood was sucked dry. She was stabilising her with one of her gems when Shirou noticed a glint at the corner of his eye.

His eyes widened.

"Tohsaka, watch out!"

Hand outstretched, Shirou clenched his eyes shut, bracing for the pain. Tohsaka screamed in alarm, but the pain never set.

Slowly, Shirou opened his eyes, only to gasp at what he saw.

There, standing in front of them protectively, was a teenage girl garbed in familiar kimono and haori. Her long dark hair with vermilion tips flowed softly in the wind, as she doubled over in pain.

Instantly, Shirou knew who it was.

"N-Nezuko!" He ran to catch her fall. Nezuko writhed, blood dripping from her bamboo gag as a semi-transparent weapon was stuck on her stomach.

"Wha - Why is she here?!" Rin rushed towards her and pressed on the wound to minimise blood loss. "Shit! This much blood...It must've hit her artery - Wait, where are you going, Shirou?"

"The one who did this is still here," Shirou gritted his teeth and rushed towards the woods, with only a reinforced pipe in his hand. "They did this to Nezuko…"

_It should've been me_.

He managed to dodge the first, second, third strike. It was a Servant, a voluptuous woman with long light purple hair and a blindfold. She used a long dagger attached to a chain, constantly throwing its tip towards him in an attempt to skewer him to death.

"I'm surprised," the Servant said. "Are you not going to summon your Servant?"

"He'll be here soon, I knew it," Shirou replied. Nezuko was injured and he couldn't do anything about it, but Saber had told him that he could sense his sister's distress much more clearly than his own Master's, because a Noble Phantasm is linked directly to the Hero's soul. They had decided to let Nezuko accompany him to school since as an independent, sentient Noble Phantasm, Nezuko could astralise at will.

Why hadn't Nezuko interfered when Tohsaka bombarded him with Gandr, he didn't know. Maybe she recognised her as a non-threat.

'No more stray thought,' Shirou told himself. 'Focus.' He inhaled deeply, entering the state of Total Concentration Breathing.

"In that case," suddenly, the Servant appeared beside him. "I should kill you quickly before he comes."

'As expected of a Servant, her hits are tough!' The blows they exchanged made him realise their difference in strength, despite him already using the Breathing Technique. Gritting his teeth, Shirou whirled to dodge the dagger, before jumping to spin in the air.

'_Breath of Water, Second Style_,' Shirou exhaled sharply as he imagined a stream of water emerging from his reinforced pipe. '_Water Wheel!_'

Surprised, the Servant barely blocked the attack. She wrapped her chain around his pipe to yank it off, but Shirou was already moving by then.

'_Breath of the Sun, Third Style_,' water shifted to fire, as he vanished from his opponent's eyes. Silent inhale. '_Fake Rainbow!_'

_I have to keep going, keep pushing, don't let her follow -_

'_Breath of the Sun, Sixth Style_,' he thrust the sharp end of his pipe towards the back of his enemy, exhaling sharply, '_Sunflower Thrust -_'

A kick in his periphery.

Sharp inhale. Muscles tensed -

_Swing!_

His pipe blocked the kick successfully. Shirou gritted his teeth at the strength, before gasping when his back hit the ground hard.

He coughed violently, struggling to let air into his lungs. His whole body hurt and his arms numb, not used to performing consecutive Breathing Styles. Distantly, he vaguely saw the enemy Servant approaching him, before a sudden gust of wind forced his eyelids to close.

A loud thud. A hacking cough. A beastly howl.

"Shirou!" Suddenly, Tohsaka appeared in front of him. He could feel her hands helping him to sit up to let him breathe easier. "Emiya-kun! You alright?!"

"T-Tohsaka?" Shirou gasped. "N-Nezuko, what about -?"

"She's fine. She has rapid regeneration," Tohsaka said curtly, looking past him. Shirou turned his head to look in the same direction.

It was bizarre to see Nezuko in her most demonic form. A horn protruded from the left side of her forehead, as veins bulged around her eyes. Her nails turned sharp, clawing and drawing blood from the Servant's skin. The bamboo gag was gone, revealing rows of sharp teeth snapping dangerously at the enemy's chain.

Nezuko roared as she lifted her leg and kicked the enemy Servant. The Servant soared a couple of meters away, blasting past trees. Suddenly, she was yanked back towards Nezuko, who held the end of the chain in her hand.

But the Servant was still the stronger of the two. She'd finally recovered from her shock, whirling to deliver a devastating kick as the dagger soared to penetrate Nezuko's shoulder.

"Nezuko!"

The Noble Phantasm was slammed to the ground, the impact so great it formed a crater. Nezuko laid still before the Servant.

"That was unexpected," the Servant said idly. She lifted her dagger, aimed to severe Nezuko's head -

And suddenly, the forest was set ablaze.

"That is enough, Rider" a deep voice resounded.

Shirou breathed a sigh of relief when his Servant finally arrived on the scene. Saber had his Nichirin sword drawn, its blade glinting reddish hue. He stood protectively in front of his downed sister, who was gradually getting smaller.

Shirou belatedly realised that the enemy Servant was missing one arm.

The wound was gruesome, flesh cauterised by the hot blade of the Demon Slayer. The Servant, Rider, was panting in exertion, not expecting the sudden arrival of the swordsman. There was a high-pitch yell from somewhere and Rider stiffened, glowing momentarily before she vanished.

"Archer, find out where she went," Tohsaka ordered her own Servant. "Judging by the yell just now, her Master must be nearby."

Shirou could hear no response, but the sudden gust of wind from behind him was telling enough.

After checking Nezuko over to make sure she was fine, Saber stood up and said, "I'll follow him, Master."

Shirou nodded, and the swordsman left.

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* * *

Honestly, Rin shouldn't have been surprised.

Archer had come back alone at first, looking absolutely exasperated. He told them he'd escort her and Shirou to the Emiya residence, where Saber had promised to see them there. Perhaps, Saber had killed the enemy Servant since the swordsman had looked livid when he cut off Rider's arm.

Of course, Kamado Tanjirou had to defy logic, because there, sitting in seiza with her arm being tended by a fretting Sakura, was _the _enemy Servant. Alive and kicking.

"Why is she here?!" Rin shrieked.

Saber sheepishly gave her sleeping Nezuko to calm her down. "You see, apparently Sakura-chan is Rider's true Master, so…"

_Palm, meet face._

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* * *

That night, Shirou punched Shinji in the face. He did it with his Total Concentration Breathing activated.

Shinji had to be brought to the hospital.

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* * *

**First part of the shot done!**

**I've written a bit for the next part, but I'm interested to hear what you guys think first.**

**Author Notes:**

**(1) In this story, Shirou (and by extension, Archer) is a descendant of Kamado Tanjirou. Only in this timeline that Shirou's body could remember the Breathing techniques his parents had taught him before the fire. Archer was a different Shirou from a different timeline, and he'd never learned the same Breathing** te**chniques as Shirou did.**

**(2) Archer did not know who his ancestor was until he met Tanjirou in Day 1. He could read the history of any sword just by a glance, but he can't see the future of the sword's wielder. Besides, Tanjirou's sword may or may not have been passed down to the next generation. Essentially, the history between Tanjirou's death and the start of the Fifth Holy Grail War was not recorded in his Nichirin sword.**

**(3) Tanjirou is aged up. Probably around his early twenties.**

**(4) Nezuko is an independent, sentient Noble Phantasm. Fight me.**

**(5) I would say Breathing techniques/styles do not perform in the same way as magecraft in FSN. Ordinary people could become Demon Slayers.**

**(6) Tanjirou inherited his hard forehead from his mother. It's genetic.**

**The strong sense of smell is genetic.**

**'Saving-people-thing' is genetic too, somewhat.**

**(7) In the scene where Shirou confronted Rider, I forgot that in the anime, Nezuko can't go out when the sun is up. But I've already written everything before I realised this. So, uh, manga spoiler? **

**(8) Why didn't Nezuko help Shirou when he was running away from Rin? Why didn't Archer help Rin kill Shirou? It's because they were both busy playing and cuddling on the school rooftop. (Just kidding.)**

**(9) How could Shirou performed Breath of Water and Breath of Sun?! In a way, it's similar to what had happened to Tanjirou when he fought Rui (the spiders). Tanjirou could recall the Dance his father had taught him when he was young. Tanjirou later on taught his own children the Breath styles early, because apparently most Demon Slayers died young. Shirou's parents did the same, though the training might've been incomplete due to their sudden deaths.**

**Besides, in Day 2 Tanjirou said he'd teach Shirou some stuff.**

**(10) Tanjirou may have accidentally suggested to Shirou and the girls that harem is okay. Hey, Tengen Uzui had _three_ wives and it turned out really well. (Just kidding too.)**

**(11) Nezuko is the ultimate cuddler. Her cuddling rank is EX.**

**(12) Why are you suddenly publishing this fic instead of updating your other ones?! Honestly I wrote this right after reading the entire KnY manga. It was such a masterpiece that I could not _not_ write a fic about it. Forgive me for being inconsistent.**

**Thanks for reading!**


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